Where's these rules listed?

[dsockrider]

On my QA review Bob Crisler said my banners need to be a max of 940px wide, but on this page it 'recommends' 930 px wide. http://wdn.unl.edu/documentation/images.shtml

Where are the official rules concerning image width?

[acoleman]

There's really no hard and fast rule, simply because some people will be using images with the CSS styled border, and some won't.

Looking at most of your pages, it seems like you'd have no use for that border, so 940px is the size that you'd want to go. The full-width images on the image styling examples page that you linked are 930px, because they have to account for the 10px worth (5px on each side) of padding and border on the "frame" style.